1. Field of the Invention
The present invention relates in general to centrifugal multiblade fans constructed of plastics, and more particularly to centrifugal multiblade fans for an automotive air conditioning device. More specifically, the present invention is concerned with the centrifugal multiblade fans of a type which is installed in an upstream section of an air duct of the automotive air conditioning device to generate, upon rotation of the fan, an air flow in the air duct toward the passenger cabin of the vehicle.
2. Description of the Prior Art
In automotive air conditioning devices, there is usually employed a centrifugal multiblade fan which is installed in an upstream section of an air duct of the air conditioning device. The fan is driven by an electric motor. That is, upon operation of the motor, the fan is rotated to generate an air flow in the air duct from the outdoor or indoor of an associated motor vehicle toward the passenger cabin of the vehicle. During the flow in the duct, the air passes through an evaporator and/or heater core to adjust temperature thereof to a desired degree. The air thus adjusted in temperature is led into the passenger cabin of the vehicle through air blow openings provided at a downstream end of the air duct.
Some of conventional fans of the centrifugal multiblade type are shown in Japanese Patent First Provisional Publications 63-97899 and 60-60299, two of which are schematically shown in FIGS. 33 and 34 of the accompanying drawings, respectively.
As shown, each fan 1a or 1b comprises a plurality of curved blades 2 which are circularly arranged about a common rotation axis at evenly spaced intervals defining an air flow passage 3 between every neighboring curved blades 2. As is seen from FIG. 35, each curved blade 2 of the fan 1a or 1b has a generally semi-cylindrical shape, that is, the shape having an arcuate cross section. That is, each curved blade 2 has concave front (or leading) and convex rear (or trailing) surfaces 4 and 5 which extend longitudinally in parallel with the rotation axis. Accordingly, each air flow passage 3 curves as it extends radially.
The terms "front and rear" are to be understood with respect to the direction ".alpha." in which the fan 1a or 1b rotates under operation of the air conditioning device. When, upon energization of the electric motor, the fan 1a or 1b is rotated in the direction of the arrow ".alpha.", air in each air flow passage 3 is forced to move radially outward due to a centrifugal force generated therein. As a result, air is forced to flow radially outward from the inside of the fan 1a or 1b toward the outside through the air flow passages 3.
As is known, the fans of the above-mentioned centrifugal multiblade type produce less operation noise than axial fans such as propeller fans. However, sometimes, even the centrifugal multiblade fans fail to provide users with full satisfaction. That is, when the air conditioning device is in an inner air circulation mode wherein the air blown to the interior of the vehicle is fed from the interior of the vehicle, the noise caused by the fan becomes noticeable, which makes the passengers uncomfortable.
In addition to the above-mentioned fans, other conventional centrifugal multiblade fans have been proposed, which are shown in Japanese Patent First Provisional Publications 62-291498 and 4-203395, and Japanese Utility Model First Provisional Publications 50-76407, 50-76408, 52-2612, 58-94898 and 5-87295. In the fans of these publications, a slit or small opening is formed in each curved blade to improve the performance of the fans. However, even the fans of these publications have failed to provide users with a satisfaction in noise reduction performance, particularly in the inner air circulation mode of the air conditioning device.